The present invention relates to gas permeable pallets for supporting a burden in traveling grate machines. More particularly, my invention relates to a pallet construction which is suitable for use in circular traveling grate machines in which one or both of the rotating side frames drive the pallets and wherein thermal expansion and contraction imposes complex dimensional variations within the structure.
Traveling grate machines are relative large and usually designed to handle high tonnage rates of particulate materials in a variety of thermal processes. Such machines may find use in the coking or gasification of coal; heat hardening or prereduction of iron ore pellets; and the retorting of oil shale, to mention a few. The traveling grate machines are characterized by a plurality of pallets, having gas permeable grates, which are moveable along a trackway while supporting a burden through the various process zones of the machine. At the outset, the pallet grates are charged with crushed, balled, pelletized, or otherwise formed and sized particulate material, such as oil shale, coal, iron ore or the like. The pallets and the supported burden are at a relatively cool temperature while in this charging zone. The charged burden is moved slowly by the pallets along the trackway to the high temperature process zones wherein hot gases are passed through the burden and the pallets in an updraft or downdraft mode or an alternating combination of the two. Upon completion of the thermal processing steps, the sensible heat remaining in the burden is oftentimes recovered by passing a cool, quenching gas through the burden and the pallet grates, which yields a cooled spent burden along with a progressing train of pallets which have also been cooled by the quenching gas. The processed, or otherwise spent, burden is then dumped from the pallets and a fresh burden is charged to begin the process cycle once again. Thus, it can be appreciated that the pallets are subjected to a wide variety of temperature and thermal gradients throughout the various zones of the traveling grate machine which necessarily causes similar dimensional variations in the pallets due to the resultant thermal expansion and contraction. These dimensional variations in the pallets are made more difficult to accommodate since the pallets are attached at their lateral edge portions to the relatively cooler, rotating side frames of the traveling grate machine. These side frames are thermally insulated from the process zone and, therefore, do not expand and contract to the same degree as the pallets and, thus, considerable dimensional variations must be dealt with. If these thermal expansion and contraction characteristics are not properly accommodated, the pallets mechanically bind, causing undue wear and/or structural failure with resultant machine stoppage. At the same time, the dimensional tolerances or spacing gaps between adjacent pallets cannot be unreasonably high so as to cause excess tunnel gas leakage between the vertical sides of the moving pallets. Such a condition results in additional gas flow through the pallet wheel tunnels which necessarily increases the size and cost of the air handling system. Thus, it is desirable to maintain a static seal along the lateral gaps between the trailing and the leading ends of the adjacent pallets while still avoiding any likelihood of binding therebetween.
It is further advantageous to employ a pallet which, when attached to the rotating side frames, is able to transmit forces and moments from one side frame to the other. This is, of course, necessary when only one of the side frames is being driven, in which case, the pallets must transmit a driving force and moment to the non-driven side frame. However, even in those cases where both of the side frames are motorized, there are periodic, momentary load mismatches between the moving side frames, since perfect load matching is difficult to achieve in these large structures. When such load mismatching across the side frames occurs. The attached pallets must be capable of transmitting and coupling the resultant forces and moments imposed thereon without undue wear or failure.
It is also a recongized condition in traveling grate machines that over a period of time, the pallets may begin to exhibit a transverse sag, possibly due to prolonged exposure to loading at high temperatures and the related thermal cycling inherent in the process. As a result, binding problems may occur in the pallet axle shafts and conventional shaft bearings of the hanger attachments when the pallets tiltably pivot in the discharge zone of the machine. Costly maintenance and expensive pallet replacement, with attendant machine downtime, can result from this thermal sagging problem.
My present invention solves many of the problems heretofore encountered in circular traveling grate machines of this type, by providing a pallet construction that accommodates the differential expansion and contraction characteristics of the pallets and the side frames of the circular traveling grate machine caused by the cyclical thermal environment imposed on the structure.
The invention further provides a pallet construction which structurally interconnects the rotating inner and outer side frames and transmits forces and moments between the rotating side frames of the traveling grate machine without binding and undue wear. The invention provides a pallet construction having selectively restrained and floating connection means which structurally interconnects the rotating inner and outer side frames of the traveling grate machine to couple forces and moments therebetween. In installations where only one of the side frames is equipped with driving means, the pallets will transmit the driving forces to the other side frame. In those cases where both of the side frames are being driven independently, a coupling action of the pallets accommodates any load mismatching which may periodically occur between the side frames.
My invention still further provides a pallet construction having hanger bracket bushings or bearings which accommodate thermal sag in the pallets, whereby service life of the pallets and hanger brackets is markedly increased.
The pallet construction of the present invention also contains sealing means which provides vertical side seals between the leading and trailing ends of adjacent pallets which results in minimum tunnel gas leakage therebetween.
In addition, my invention provides a pallet construction that includes a lubrication system which permits the lubrication and flushing of the hanger bracket axle bushings or bearings and the rear wheel bearings on opposed sides of the pallet by way of a lubrication port on each side of the pallet at an accessible location, spaced from the heat of the process zone of the machine.